[aprssig] differential corrections available to public (in some places)

[Robert Bruninga]

>>> jimlux at earthlink.net 3/13/05 2:50:18 PM >>>
>Some thought should be given how to put these [differential]
>corrections on the 2m air in a useful fashion and for
>future GPS/TNC widgets to accept corrections and feed 
>them to the GPS.

We were doing this back in 1995 time frame in APRS in
the Washington DC area and APRSdos has the hooks 
built-in.  I have not looked at it in years, but It is documented 
towards the end of the  APRSdos README\GPS.TXT file.

That may all be old or obsolete, but it is how we were
recommending it back then.  I think we were suggesting
that the RTCM104 be on a different 2m freq so that it
couild be real-time full operations without any QRM to 
144.39.

de WB4APR, Bob


One might want to take a look at: http://www.mdotcors.org/ for the
state of
Michigan, as an example, although I couldn't find any info on RTCM104
corrections there, but they do give Rinex format data for post
processing.

Here's another interesting source of differential correction data:
http://www.wsrcc.com/wolfgang/gps/dgps-ip.html 
http://www.rlageosystems.com/dgps.htm has many links to data, not sure
which
have realtime data available.

Anyway, folks who have a suitable receiver and a fixed station
location
could do real good for those near them by setting up to put RTCM104
corrections out there.  Bob? Gerry? Suggestions on how to do this
within
APRS without overflowing the capacity.  I don't know enough about
differential correction to know how often you want to push corrections
out,
and how to manage the distance issue (you want corrections from
someone
close to you, but, on the other hand, for APRS kinds of applications,
we're
probably not interested in centimeter accuracy.. meter is good
enough).

And, since a lot of people have given the whole prospect of
broadcasting GPS
corrections over wide areas a fair amount of thought, reports such as
http://www.lvvwd.com/assets/pdf/wolftuc99_low_res.pdf might save a bit
of
"reinventing the wheel" or "inventing a new incompatible wheel"
problems.

Jim, W6RMK


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